As technological development and demand for mobile devices increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate are commercially available and widely used. Further, to make an electrode for such a high-capacity lithium secondary battery, research on methods for producing an electrode having a higher energy density per unit volume by improving electrode density has been actively conducted.
Generally, since a high-density electrode is formed by molding electrode active material particles having a size of several μm to several tens of μm using a high-pressure press, particles are deformed, the space between the particles is reduced, and electrolyte permeability may be lowered.
In order to solve such a problem, a conductive material having excellent electrical conductivity and strength is used in the production of electrodes. When a conductive material is used in the production of electrodes, the conductive material is dispersed among the compressed electrode active material, and thereby fine pores are maintained between the active material particles to facilitate the penetration of the electrolyte, and the resistance in the electrode can be lowered due to excellent conductivity. Among such conductive materials, the use of carbon nanotubes, which are fibrous carbon-based conductive materials, capable of further reducing electrode resistance by forming an electrically conductive path in the electrode has been increasing.
The carbon nanotube, which is a type of fine carbon fiber, is tubular carbon having a diameter of 1 μm or less, and is expected to be applied and practically used in various fields due to high conductivity, tensile strength, and heat resistance attributable to the specific structure thereof. However, despite the availability of such carbon nanotubes, the use of carbon nanotubes is limited due to low solubility and dispersibility thereof. Accordingly, in the preparation of electrodes using carbon nanotubes, carbon nanotubes are linearly dispersed in a dispersion medium, and then a composition for forming an electrode is prepared and used. However, carbon nanotubes do not stably disperse in the dispersion medium, and an aggregation phenomenon occurs due to the strong van der Waals attraction between the carbon nanotubes.
Various attempts have been made to address these issues. Specifically, a method of dispersing carbon nanotubes in a dispersion medium through mechanical dispersion treatment such as ultrasonic treatment has been proposed. However, in this method, dispersibility is excellent while the ultrasonic wave is irradiated, but the carbon nanotubes start to aggregate when the ultrasonic irradiation is finished. Further, a method of dispersing and stabilizing carbon nanotubes using various dispersants has been proposed. However, this method also has a problem in that, when carbon nanotubes are dispersed at a high concentration in a dispersion medium, handling becomes difficult due to an increase in viscosity.
Accordingly, there is a need for development of a method capable of improving the dispersibility of carbon nanotubes in an electrode without reducing conductivity, and a method of preparing a carbon nanotube dispersed liquid which is useful in the production of electrodes using the above-described method.